If you want to go by actual numbers, your truck empty, rear axle is 3000-3500lbs.
Your back tires are good for somewhere between 7200 and 8000lbs depending on the tire on OE type rims. OE Rims are generally not published but are rated for 3500-3600lbs each. Axle is rated for 10,000lbs.
Axle is definitely overbuilt for whatever you load up, so is the frame and spring mounts.
My belief is rims have a significant factor of safety as well.
With the proper add ons I would be comfortable with 4000-4500lbs total load on the rear axle, all day long. What the legality of that is in your province or your comfort level is, I cannot say.
